Normal sinus rhythm of the heart begins with the sinoatrial node (or “SA node”) generating an electrical impulse. The impulse usually propagates uniformly across the right and left atria and the atrial septum to the atrioventricular node (or “AV node”). This propagation causes the atria to contract in an organized manner to transport blood from the atria to the ventricles, and to provide timed stimulation of the ventricles. The AV node regulates the propagation delay to the atrioventricular bundle (or “HIS” bundle). This coordination of the electrical activity of the heart causes atrial systole during ventricular diastole. This, in turn, improves the mechanical function of the heart. Atrial fibrillation occurs when anatomical obstacles in the heart disrupt the normally uniform propagation of electrical impulses in the atria. These anatomical obstacles (called “conduction blocks”) can cause the electrical impulse to degenerate into several circular wavelets that circulate about the obstacles. These wavelets, called “reentry circuits,” disrupt the normally uniform activation of the left and right atria.
Because of a loss of atrioventricular synchrony, people who suffer from atrial fibrillation and flutter also suffer the consequences of impaired hemodynamics and loss of cardiac efficiency. They are also at greater risk of stroke and other thromboembolic complications because of loss of effective contraction and atrial stasis.
One surgical method of treating atrial fibrillation by interrupting pathways for reentry circuits is the so-called “maze procedure,” which relies on a prescribed pattern of incisions to anatomically create a convoluted path, or maze, for electrical propagation within the left and right atria. The incisions direct the electrical impulse from the SA node along a specified route through all regions of both atria, causing uniform contraction required for normal atrial transport function. The incisions finally direct the impulse to the AV node to activate the ventricles, restoring normal atrioventricular synchrony. The incisions are also carefully placed to interrupt the conduction routes of the most common reentry circuits. The maze procedure has been found very effective in curing atrial fibrillation. However, the maze procedure is technically difficult to do. It also requires open heart surgery and is very expensive.
Maze-like procedures have also been developed utilizing catheters, which can form lesions on the endocardium (the lesions being 1 to 15 cm in length and of varying shape) to effectively create a maze for electrical conduction in a predetermined path. The formation of these lesions by soft tissue coagulation (also referred to as “ablation”) can provide the same therapeutic benefits that the complex incision patterns that the surgical maze procedure presently provides, but without invasive, open heart surgery.
One lesion that has proven to be difficult to form with conventional devices is the circumferential lesion that is used to isolate the pulmonary vein and cure ectopic atrial fibrillation. Lesions that isolate the pulmonary vein may be formed within the pulmonary vein itself or in the tissue surrounding the pulmonary vein. Ablation of pulmonary veins is currently performed by placing a diagnostic catheter (such as Biosense Webster's Lasso™ circular ECG catheter, Irvine Biomedical's Afocus™ circular ECG catheter, or Boston Scientific Corporation's Constellation™ ECG catheter) into the pulmonary vein to be treated, and then ablating the pulmonary tissue adjacent to the distal end of the selected diagnostic catheter with a standard, commercially available ablation catheter. The diagnostic catheter is used to determine if the lesion created by the ablation catheter has been successful in electrically isolating the pulmonary vein.
Some physicians may alternatively use a standard linear diagnostic catheter with 2-20 ECG electrodes to evaluate pre-ablation electrocardiogram (ECG) recordings, then swap the diagnostic catheter with a standard ablation catheter either through the same sheath, or in conjunction with the ablation catheter through a second sheath, ablating the pulmonary tissue, and then swapping the ablation catheter with the diagnostic catheter to evaluate post-ablation ECG recordings.
In any event, the circumferential lesion must be iteratively formed by placing the ablation electrode into contact with a tissue region, ablating the tissue region, moving the ablation electrode into contact with another tissue region, and then ablating again. In a standard procedure, placement of the electrode and ablation of tissue may be repeated from 15-25 times to create the circumferential lesion. It is often difficult to form an effective circumferential lesion, however, by forming a pattern of relatively small diameter lesions. More recently, inflatable balloon-like devices that can be expanded within or adjacent to the pulmonary vein have been introduced. Although the balloon-like devices are generally useful for creating circumferential lesions, these devices have the undesirable effect of occluding blood flow through the pulmonary vein.
In response to these problems, a corkscrew-type ablation catheter has been recently designed. This catheter comprises a helical distal end on which a plurality of ablation electrodes are mounted. The helical distal end can be inserted into a pulmonary vein to be treated and operated to efficiently produce a circumferential lesion, while allowing passage of blood. The ablation electrodes on the corkscrew-type ablation catheter can also be used to generate ECG recordings as a frame of reference for the ablation procedure. In use, it has been noted that ECG drops in amplitude are an indicator of potential pulmonary vein electrical isolation. Although this technique has proven successful, the ablation device does not offer as high of an ECG signal resolution as would a dedicated ECG catheter.
Accordingly, there remains a need to be able to more efficiently create circumferential lesions and provide high resolution ECG recordings within or around bodily orifices without occluding fluid flow and, in the context of the treatment of atrial fibrillation, within or around the pulmonary vein without occluding blood flow.